Download File

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 13
How Populations Evolve
T. Dobzhansky: Nothing in biology makes sense
except in the light of evolution.
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Clown, Fool, or Simply Well Adapted?
•  All organisms have evolutionary adaptations
–  Inherited characteristics that enhance their ability to
survive and reproduce
–  Further observation often reveals that an organism’s
features show some relationship to where the organism
lives and what it does in its environment.
•  The blue-footed booby of the
Galápagos Islands has features
that help it succeed in its
environment
–  Large, webbed feet help
propel the bird through
water at high speeds
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– A streamlined shape, large tail, and nostrils that
close are useful for diving
– Specialized salt-secreting glands manage salt
intake while at sea
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
EVIDENCE OF EVOLUTION
13.1 A sea voyage helped Darwin frame his theory
of evolution
•  Anaximander suggested that life arose in water
and simpler forms preceded more complex
forms of life.
•  Aristotle and the Judeo-Christian culture
believed that species are fixed
•  Fossils suggested that life forms change
– This idea was embraced by Lamarck in the early
1800s
– Used giraffes as an example
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  While on the voyage of the HMS Beagle in the
1830s, Charles Darwin observed
– similarities between living and fossil organisms
– the diversity of life on the Galápagos Islands,
such as blue-footed boobies and giant tortoises
Figure 13.1A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  The voyage of the Beagle
Great
Britain
Europe
North
America
Pacific
Ocean
Atlantic
Ocean
Africa
Galápagos
Islands
Equator
South
America
Andes
Australia
Cape of
Good Hope
Cape Horn
Tierra del Fuego
Tasmania
New
Zealand
Figure 13.1B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  Darwin became convinced that the Earth was
old and continually changing
– He concluded that living things also change, or
evolve over generations
– He also stated that living species descended
from earlier life-forms: descent with
modification
•  Darwin’s On the Origin of Species was
published in 1859
– Uses the phrase “descent with modification”
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.2 The study of fossils provides strong evidence
for evolution
•  Fossils and the fossil record
strongly support the theory of
evolution
– Hominid skull
– Petrified trees
Figure 13.2A, B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Ammonite casts
– Fossilized organic
matter in a leaf
Figure 13.2C, D
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Scorpion in amber
– “Ice Man”
Figure 13.2E, F
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  The fossil record shows that
organisms have appeared in a
historical sequence
•  Many fossils link
early extinct species
with species living
today
– These fossilized
hind leg bones link
living whales with
their land-dwelling
ancestors
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 13.2G, H
13.3 A mass of evidence validates the evolutionary
view of life
•  Other evidence for evolution comes from
– Biogeography
•  Geographic distribution of species
– Comparative
anatomy
•  Anatomical similarities
give signs of common
descent.
– Comparative
embryology
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Human
Cat
Whale
Bat
Figure 13.3A
– Molecular biology
Human
Rhesus monkey
Last common
ancestor lived
26 million years
ago (MYA),
based on
fossil evidence
Mouse
Chicken
Frog
Lamprey
80 MYA
275 MYA
330 MYA
450 MYA
Figure 13.3B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
DARWIN’S THEORY AND THE MODERN
SYNTHESIS
13.4 Darwin proposed natural selection as the
mechanism of evolution
•  Darwin observed that
– organisms produce more offspring than the
environment can support
– organisms vary in many characteristics
– these variations can be inherited
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  Darwin concluded that individuals best suited
for a particular environment are more likely to
survive and reproduce than those less well
adapted
•  Darwin saw natural selection as the basic
mechanism of evolution
– As a result, the proportion of individuals with
favorable characteristics increases
– Populations gradually change in response to the
environment
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  Darwin also saw that when
humans choose organisms
with specific characteristics as
breeding stock, they are
performing the role of the
environment
– This is called artificial
selection
– Example of artificial
selection in plants: five
vegetables derived from
wild mustard
Figure 13.4A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Example of artificial selection in animals: dog
breeding
German shepherd
Yorkshire terrier
English springer
spaniel
Mini-dachshund
Golden retriever
Hundreds to
thousands of years
of breeding
(artificial selection)
Ancestral dog
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 13.4B
•  These five canine species evolved from a
common ancestor through natural selection
African wild
dog
Coyote
Fox
Wolf
Jackal
Thousands to
millions of years
of natural selection
Ancestral canine
Figure 13.4C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.5 Connection: Scientists can observe natural
selection in action
•  Evolutionary adaptations have been observed
in populations of birds, insects, and many other
organisms
– Example: camouflage adaptations of mantids
that live in different environments
Figure 13.5A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  The evolution of insecticide resistance is an
example of natural selection in action
Insecticide
application
Chromosome with gene
conferring resistance
to insecticide
Additional
applications of the
same insecticide will
be less effective, and
the frequency of
resistant insects in
the population
will grow
Survivor
Figure 13.5B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.6 Populations are the units of evolution
•  A species is a group of populations whose individuals
can interbreed and produce fertile offspring
•  A population is a group of individual organisms living
in the same place at the same time.
–  Human populations tend
to concentrate locally, as
this satellite photograph
of North America shows
•  The modern synthesis
connects Darwin’s theory
of natural selection with
population genetics
Figure 13.6
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.7 Microevolution is change in a population’s
gene pool over time
•  A gene pool is the total collection of genes in a
population at any one time
•  Microevolution is a change in the relative
frequencies of alleles in a gene pool
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.8 The gene pool of a nonevolving population
remains constant over the generations
•  Hardy-Weinberg equilibrium
states that the shuffling of
genes during sexual
reproduction does not alter
the proportions of different
alleles in a gene pool
– To test this, let’s look at an
imaginary, nonevolving
population of blue-footed
boobies
Webbing
No webbing
Figure 13.8A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  The Hardy-Weinberg equation shows that
allele frequencies are stable in a population not
undergoing microevolution: p2 + 2pq + q2 = 1.
– The population is made up of: homozygous
dominant genotypes (p2) + heterozygous
genotypes (2pq) + homozygous recessive
genotypes (q2).
– Also note that p + q = 1
Figure 13.8B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  We can follow alleles in a population to observe
if Hardy-Weinberg equilibrium exists
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320/
500
= 0.64
Number of alleles
in gene pool
(total = 1,000)
640 W
Allele frequencies
800/
1,000
160/
500
= 0.32
160 W + 160 w
= 0.8 W
200/
1,000
20/
500
= 0.04
40 w
= 0.2 w
Figure 13.8B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Recombination
of alleles from
parent generation
SPERM
EGGS
WW
2
p = 0.64
WW
qp = 0.16
Ww
pq = 0.16
ww
q2 = 0.04
Next generation:
Genotype frequencies
0.64 WW
Allele frequencies
0.32 Ww
0.8 W
0.04 ww
0.2 w
Figure 13.8C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.10 Five conditions are required for HardyWeinberg equilibrium
•  The population is very large
•  The population is isolated
•  Mutations do not alter the gene pool
•  Mating is random
•  All individuals are equal in reproductive success
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.11 There are several potential causes of
microevolution
•  Genetic Drift can change the gene pool due to
random unselective forces
•  Gene flow can change a gene pool due to the
movement of genes into or out of a population
•  Mutation changes alleles
•  Nonrandom mating: May or may not change
allele frequency. Therefore may or may not be a
source of microevolution.
•  Natural selection leads to differential
reproductive success
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.11 There are several potential causes of
microevolution
•  Genetic drift is a change in
a gene pool due to chance
–  Genetic drift is most likely
to occur when something
happens to reduce the
population size
•  Bottleneck Effect
•  Founder Effect
–  Due solely to chance
–  Strongest affect on small
populations
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Original
population
Bottlenecking
event
Surviving
population
Figure 13.11A
–  Bottle Neck Effect: Northern Elephant Seals are
genetically identical at all gene loci studied so far (24)
due to a bottleneck caused by hunting
–  Founder Effect: This British colony has a high
incidence blindness caused by a recessive genetic
disorder. Evidently one of the original colonists carried
a single recessive allele.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.12 Adaptive change results when natural
selection upsets genetic equilibrium
•  Natural selection results in the accumulation of
traits that adapt a population to its environment
– If the environment should change, natural
selection would favor traits adapted to the new
conditions
– Diversity within a population allows populations
to adapt when the environment changes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
VARIATION AND NATURAL SELECTION
13.13 Variation is extensive in most populations
•  Natural Selection favors some alleles more than others,
but it works by selecting specific traits caused by these
alleles; by selecting the phenotype, not the alleles.
•  Phenotypic variation may be environmental or genetic
in origin
–  Only genetic changes result in evolutionary adaptation
because environmental variation is not heritable.
–  Not all genetic Variation is subject to natural selection.
Neutral variations provide no selective advantage
regardless of phenotype. Ex - fingerprints
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•  Many populations exhibit polymorphism and
geographic variation (clines)
– Polymorphism: two or more different forms of a
phenotypic trait
– Cline: a graded change in the frequency of an
inheritable trait over a species geographic range
Figure 13.13
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Measures of Genetic Diversity Within a Population
•  Gene Diversity
– Average # of gene loci that are heterozygous in a
population. 14% for humans.
•  Nucleotide Diversity
– Average % difference in nucleotide sequences
between members of a population.
– 0.1% for humans.
•  Not much, but enough to create all of the diversity
you see in the human population.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.14 Connection: Mutation and sexual
recombination generate variation
•  Mutations are normally
harmful, but they may
improve an organism’s
adaptation to an
environment that is
changing.
• Organisms with very short
generation spans and
haploid, RNA genomes, like
viruses, can evolve rapidly
by mutation alone.
Parents
A
1
A
A
1
A
2
3
MEIOSIS
A
Gametes
A
2
3
A1
FERTILIZATION
Offspring, with new
combinations of
alleles
A
1
A
2
and
A
1
A
3
• Eukaryotic organisms rely more on sexual reproduction for variation.
New assortments of alleles arise every generation due to crossing over,
independent assortment, and random fertilization.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.15 Overview: How natural selection affects
variation
•  Natural selection tends to reduce variability in
populations, but may also lead to balanced
polymorphism:
– The diploid condition preserves variation by “hiding”
recessive alleles
– For some codominant or incomplete dominant
traits, heterozygotes may be favored by the
environment and thus two or more alleles for the
trait are preserved. (heterozygous advantage)
– Frequency Dependent Selection promotes variation
by increasing selection against the more common
phenotypes.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.17 Connection: Endangered species often have
reduced variation
•  Low genetic variability may reduce the capacity
of endangered species to survive as humans
continue to alter the environment
– Studies have shown that cheetah populations
exhibit extreme genetic uniformity
– Thus they may have a
reduced capacity to
adapt to environmental
challenges
Figure 13.17
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.18 The perpetuation of genes defines
evolutionary fitness
•  An individual’s Darwinian fitness is the
contribution it makes to the gene pool of the
next generation relative to the contribution
made by other individuals
•  In the context of evolution, the fittest
individuals are those that pass on the greatest
number of genes to the next generation
•  Production of fertile offspring is the only score
that counts in natural selection
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.19 There are three general outcomes of natural
selection – Stabilizing Selection
Frequency of
individuals
Original
population
Evolved
population
Original
population
Phenotypes (fur color)
Stabilizing selection
•  Stabilizing Selection favors intermediate variations for a trait
among a population.
•  Typically occurs in stable environments, with limited changes.
•  Example: Human Birth weight
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Frequency of
individuals
13.19 There are three general outcomes of natural
selection
Original
population
Phenotypes (fur color)
Directional selection
•  Directional selection favors individuals with one extreme
variation.
•  Most common during periods of environmental change.
•  In the mouse population above, if the environment became darker
as a result of pollution, darker mice would be favored.
• Example: Insects and in fields recently treated with insecticide.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Frequency of
individuals
13.19 There are three general outcomes of natural
selection
Original
population
Phenotypes (fur color)
Diversifying selection
• Diversifying selection favors individuals with either extreme of a
phenotypic range among a population. It selects against
intermediate phenotypes.
•  Typically occurs in varied but stable habitats.
•  In the mouse population above, if the habitat consists of dark
rock patches, and light sandy patches, then selection would favor
light and dark mice and select against medium colored mice.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.20 Sexual selection may produce sexual
dimorphism
•  Sexual selection leads to the evolution of secondary
sexual characteristics and sexual dimorphism.
– These may give individuals an advantage in mating
– Example: males competing for privilege to mate with
female
Intrasexual
Selection
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Intersexual
Selection
13.21 Natural selection cannot fashion perfect
organisms
•  There are four reasons for imperfections in spite of
natural selection:
–  historical constraints: natural selection works with
the existing forms (phenotypes), does not start from
scratch and create new organisms
–  adaptive compromises: Organisms must be able to
do many different things. What is good for one situation
may not be the best in another
–  chance events: not all changes in the gene pool are
adaptive, chance can randomly favor some phenotypes
and thus the alleles that cause them
–  availability of variations: Works with existing alleles
or new mutations only. Can not create new ones on
demand
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
13.22 Connection: The evolution of antibiotic
resistance in bacteria is a serious public
health concern
•  The excessive use of antibiotics is leading to the
evolution of antibiotic-resistant bacteria
– Example:
Mycobacterium
tuberculosis and MRSA
Figure 13.22
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings